Method of recovering liquefiable components from a subterranean earth formation



Marsh 17,, 1970 NQFZDGREN EI'AL 3,5@Q,913 METHOD OF REGOVERING LIQUEFIABLE COMPONENTS FROM A SUBTERRANEAN EARTH FORMATION Filed 0G1.- 50, 1968 ULTIMATE COMMUNICATION l6 GAS Q AlR SEPARATOR 4 OIL um HEAT EXCHANGER INVENTORSI R. P. NORDGREN P. J. CLOSMANN THEIR ATTORNEY United States Patent U.S. Cl. 166-259 8 Claims ABSTRACT OF THE DISCLOSURE A method of recovering liquefiable components from a normally impermeable subterranean earth formation by extending at least a pair of well boreholes into the formation and forming generally vertical fractures extending along generally parallel paths from each of the boreholes. Hot fluid is injected through at least one of the well boreholes until flow into at least one of the fractures therein is thermally closed by the swelling-shut of the walls of the fracture. Fluid in at least one borehole in which fractures have been thermally closed is pressurized until at least one new fracture is formed and the steps of injecting hot fluid and pressurizing fluid are repeated at successively higher temperatures and pressures until the resultant fractures form a channel interconnecting the well boreholes through which channel fluid can flow from one well borehole to another. Finally, hot fluid is circulated, by injection into one of the boreholes opening into the fracture channel, with fluid being produced from another of the boreholes opening into the fracture channel, at a temperature below that at which the last fracture was opened in the well borehole into which the circulating fluid is injected but above that at which the last fracture was thermally closed within the well borehole into which the circulating fluid is injected, so that most of the injected fluid is conveyed through fractures interconnecting the boreholes.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method for recovering liquefiable components from a relatively impermeable subterranean earth formation, normally subjected to vertical fracturing.

Description of the prior art It is known that it is extremely difficult to recover liquefiable components from depths of various subterranean impermeable formations such as oil shale, coral, bed deposits of cinnabar, etc., under conditions in which the deposits are normally present in these formations. Various proposals have been made, such as described in a U.S. Patent No. 3,284,281, to recover oil from oil shale. Therein oil shale is produced from an oil shale formation through fractures. However, under the conditions described therein, the fractures tend to close as their walls become heated and thermally expanded and oil recovery is stopped. Under conditions described, it is generally necessary to resort to a repeating sequence of fracturing, heating and expanding and refracturing operations until flow patterns are finally formed that will remain open while the desired components are being liquefied.

In certain situations, particularly at relatively shallow depths, a heating procedure can be utilized to cause the swelling tendencies of the earth formations to create horizontal stresses that exceed the vertical stresses. By such procedure, pairs of wells can be interconnected by means of a horizontal fracture that can be kept open by hydrau- 3,500,913 Patented Mar. 17, 1970 lically lifting or compressing the overlying earth formations. Such a procedure, in which the heating is accomplished by injecting a liquid while maintaining a specified rate of fluid flow and temperature increase, is described in a copending application to Matthews et al., Ser. No. 578,533 filed Sept. 12, 1966, now Patent No. 3,455,391.

However, in many situations, horizontal fractures cannot feasibly be formed by thermally increasing the horizontal stresses. Generally, the heating of oil shale formations causes the vertical stresses to increase at a rate comparable to that at which the horizontal stresses are increased, and this prevents the formation of a horizontal fracture. Although communication between wells might be established by repetitively heating and fracturing at successively higher pressures and temperatures, as proposed in the aforementioned U.S. patent, this would result in excessively high operating costs and the production and maintenance of higher pressures and temperatures than are actually required.

When the regional tectonics are such that vertical fractures form and propagate along generally parallel paths when fluid is injected into adjacent wells at about the nor mal subsurface temperature at pressures and rates suflicient to form and extend the fractures, the use of successively higher pressures and temperatures is apt to form a succession of numerous differently-oriented vertical fractures at pressures which do not become high enough to produce a horizontal fracture. The pressure necessary to form a horizontal fracture is generally about equal to or slightly less than one p.s.i. per foot of depth. When, for example, a vertical fracture opening into a first well has been extended for a significant distance and at least one vertical fracture has been opened and then thermally closed in a second well, the formation and extension of a subsequent fracture from the second well is apt to cause the fractures to intersect so that fluid can flow from one well to the other. However, if the fluid pumped through such intersecting fractures is heated at or above the temperature at which the last fracture was formed, the walls of the fracture will swell and the flow path will close.

SUMMARY OF THE INVENTION It is an object of this invention to provide a method of recovering liquefiable components from a normally impermeable subterranean earth formation by extending generally vertical fractures between wells extending into the formation.

It is a further object of this invention to provide a method of interconnecting wells extending into normally impermeable subterranean earth formations by forming generally vertical fractures between such wells.

It is a still further object of this invention to interconnect wells extending into subterranean earth formations by means of generally vertical fractures which remain open so that fluid flow remains unrestricted between such wells.

These objects are preferably attained by forming generally vertical fractures extending along generally parallel paths from at least a pair of well boreholes extending into a relatively impermeable subterranean earth formation by injecting a hot fluid through at least one of the Well boreholes until flow into at least one of the fractures therein is thermally closed by the swelling-shut of the walls of the fracture. The fluid in at least one borehole in which fractures have been thermally closed is pressurized until at least one new fracture is formed and the steps of injecting hot fluid and pressurizing fluid are repeated at successively higher temperatures and pressures until the resultant fractures form a channel interconnecting the well boreholes, through which channel fluid can flow from one borehole to another. Finally, hot fluid is cirulated, by injecting into one of the boreholes opening no the fracture channel, with fluid being produced from nother of the boreholes opening intothe fracture chane1, at a temperature below that at which the last fracure was opened into the well borehole into which the irculating fluid is injected but above that which the last racture was thermally closed Within the well borehole 1to which the circulating fluid is injected so that most f the injected fluid is conveyed through fractures interonnecting the boreholes.

When such a flow path is opened between a pair of vells, the path may be kept open by lowering the tem- Ierature of the fluid pumped through the path to a temuerature that is less than that at which the last fracture /as formed but greater than that at which the next preeding fracture was thermally closed. In addition, if the ffective permeability of the flow path between the well oreholes is then increased by circulating through it a luid that removes solid components from the walls of he fractures, the flow path is converted to one through vhich the flow of fluid from one well borehole to the other emains eflicient at whatever temperature is subsequently rnparted to the fluid.

The attainment of a significant increase in the effective Iermeability of such a flow path may be detected by an ncrease in the rate of flow in response to the injection ressure that was initially required to displace fluid from -ne well borehole to another. If the temperature of the ;irculating fluid is too high relative to the rate at which )ermeability is being increased, this may be detected by a 'eduction in the flow rate. If the temperature is too low elative to the rate at which the permeability is being ncreased, this may be detected by a decrease in the rate )f outflow from the production well borehole without a :orresponding decrease in rate of inflow into the injection vell borehole. Where the temperature is too high, the heremal closing of the last-opened fracture throttles the low and where the temperature is too low, a thermal )pening of previously closed fractures divert the flow )f the injected fluid.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a top plan view of a preferred arrangenent of well boreholes extending into a subterranean :arth formation; and

FIGURE 2 is a vertical sectional view of the well oreholes of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, FIGURE 1 shows a air of well boreholes 11 and 12 extending into an earth Formation 13 overlying a normally impermeable suberranean earth formation such as an oil shale formation Knot shown). Generally vertical fractures are first formed :xtending along generally parallel paths from each of vell boreholes 11 and 12 which open into the subterranean :arth formation. Such fractures may be formed by any .echnique for applying a fluid pressure above the breaklown pressure of earth formation 13 but below the over- Jurden pressure of earth formation 13. Of course, tlthough two such well boreholes 11 and 12 are illustrated n FIGURE 1, obviously a plurality of such well bore- 1oles may be opened into the selected earth formation 1nd treated simultaneously or sequentially, in any order.

In a preferred procedure for interconnecting well boreioles 11 and 12, a vertical fracture 14 is extended from vell borehole 11 for a significant distance along a natural )13116 of weakness with the subterranean formation. Conentional subterranean stress analyses techniques and/or techniques for measuring the orientation of fractures may )e utilized to determine the direction along which the fracture 14 is extended. This fracture 14 is preferably propped by procedures well known in the the art. Well borehole 12, preferably having been drilled after the formation of fracture 14, is preferably completed near an intersection of a preselected well borehole-to-well borehole spacing distance and the fracture 14 opening into well borehole 11. Of course, if suflicient communication between well borehole 11 and 12 results from drilling well borehole 12 with the intention of intersecting fracture 14, further fracturing is not necessary. However, in most cases, an initial fracture 15 must be then produced in Well borehole 12 and left unpropped and thermally closed, by the method discussed in a copending application to Matthews et al. Ser No. 578,533, filed Sept. 12, 1966, wherein a liquid is pumped into a fracture within a subsurface earth formation while heating the inflowing liquid as preferably a surface location and maintaining a flow rate that is adequate for transporting heat from the heating location to the fracture. The flow rate and the rate at which the inflowing liquid is heated to temperatures preferably increasingly greater than the earth formation temperature are correlated so that the liquid remains hotter than the fracture walls until the liquid has moved a significant distance away from the well. The pressure at which the liquid is injected is increased as required in order to maintain an adequate rate of flow. The heating and pumping of the liquid are continued until the pressure at which the liquid is injected exceeds the overburden pressure and produces a horizontal fracture within the subsurface earth formation.

Thus, hot fluid may be injected into well borehole 12 until the flow into at least one of the generally vertical fractures, such as fracture 15, is thermally closed by the swelling shut of the fracture walls. Fluid in well borehole 11 in which fracture 15 has been thermally closed is then pressurized until at least one new fracture, such as fracture 17, extending from well borehole 12, is formed;

The injection of a hot fluid, such as hot water, into fracture 15 results in a gradual closing of the fractures and loss of injectivity. Continued injection of hot water, however, at suflicient pressure may cause additional vertical fractures, such as fractures 16 and/or 18 extending from well borehole 12, to form, since heating generally results in a horizontal stress comparable to or less than the vertical stress developed. Additional injection of hot water may cause fractures such as fractures 16 or 18 extending from well borehole 12, to be developed. These fractures are formed radially extending from their respective well boreholes, but, at great distances from their respective well boreholes, they generally follow some preferential alignment in the oil field. It is likely, however, that two or more of these fractures from the two well boreholes 11 and 12, for example, intersect and provide fluid communication between well boreholes 11 and 12, if injection pressure is suflicient. The use of slant well boreholes may improve chances of intercommunication.

Thus, as illustrated in FIGURE 1, these steps are repeated at successively higher temperatures and pressures to the extent required to form the interconnecting fractures, such as fractures 16, 18, and 14, through which fluid may flow from well borehole 11 to well borehole 12. Such fractures, as for example fracture 18, may be on a plane at a generally right angle to fracture 15 and thus may be extended by means well known in the art to a position intersecting fracture 14. When a subsequent fracture from well borehole 12 intersects with one from well borehole 11, as for example fracture 18 in FIGURE 1, it is propped, with the propping agent being injected while circulating fluid at a controlled temperature. This is accomplished by circulating fluid, by injection into injection well borehole 11, for example, and producing it from production well borehole 12, again for example, which opens into an interconnecting fracture, such as fractures 14 and 16. The fluid is preferably circulated at a temperature below that at which the last fracture was open into the injection well borehole 11 but above that at which the last fracture was thermally closed within the injection well borehole 11 so that most of the injected fluid is conveyed through the interconnecting fractures.

In injecting hot fluid through well borehole 12 until flow into at least one of the generally vertical fractures, such as fracture 15, is thermally closed by the swelling shut of the fracture walls, the hot fluid may be any gas and/or liquid that is heated at the surface of the earth formation 13, in the well borehole and/or in situ, e.g., by underground combustion, in the subterranean earth formation. The heated fluid may be the same as or different from the fracturing fluid and the heating may be initiated before or after the fracturing.

In circulating fluid into injection well borehole 11 and producing it from production well borehole 12, the circulation of such fluid may be continuous or intermittent and the circulated fluid may be any liquid or gas that is injected into or produced from either of any pair of well boreholes. Where one well borehole of a pair of well boreholes is fractured only once, as for example well borehole 11 of FIGURE 1, and the circulating fluid is injected through it, the temperature of the circulating fluid may be as low as desired.

Preferably, the temperature of the hot fluid being injected to form and extend a subsequent fracture in a well borehole in which a preceding fracture was thermally closed, is adjusted and a propping agent is mixed with the inflowing fluid. Once inter-well communication has been established, hot fracturing fluid injection may be discontinued and a reacting fluid may be flowed therethrough.

The composition of the reaction fluid is preferably adjusted to the extent required to circulate fluid that removes solid materials from the walls of the interconnecting fractures without a significant reduction in the average rate of flow between well boreholes 11 and 12, so that the effective permeability of the interconnecting fractures, as for example fractures 14 and 16, is increased relative to fluid flowing between well boreholes 11 and 12. Solid-materialremoving components may be incorporated into the react ing fluid being circulated through the interconnecting fractures without interrupting the flow to an extent that permits the fractures to close and reseal. In treating a subterranean oil shale formation, such components may comprise hot benzene, steam, or other solvent, or nitric acid, of a lower temperature than the hot fracturing fluid. Nitric acid has the advantage of reacting with the organic matter as well as the carbonate present in the subterranean earth formation. The injection of such a reacting fluid leaches out part of the kerogen adjoining the faces of the interconnecting fractures. The injection at a lower temperature and at substantially the same injection pressure permits the fractures to open slightly for better passage of the fluids. The temperature of the solid-material-removing fluid may be increased as the permeability of the interconnecting fractures, fractures 14 and 16, for example, is increased until the circulating fluid becomes hot enough to liquefy the liquefiable components of the subterranean earth formatron.

Following or during the hot solvent injection as discussed hereinabove, acid may be injected to react with part of the rock matrix along the fracture walls. This acid injection renders the channels even more permeable.

After all the steps discussed hereinabove are carried out, an underground combustion process, as is well known in the art, which develops considerably higher temperatures, may be undertaken. The steps of leaching out part of the kerogen and the rock generally make closure of the fracture paths during combustion very unlikely. In this manner, it is possible to treat a substantial part of the formation by underground combustion.

Thus, as illustrated in FIGURE 1, uniform temperature zones 21 and 22 may be seen surrounding well boreholes 11 and 12, respectively. Also, advancing combustion fronts 23 and 24, initiated and alternatingly advanced, for example, by means well known in the art, may be formed about well boreholes 11 and 12, respectively.

FIGURE 2 shows permeable channel 25 formed in the subterranean earth formation of FIGURE 1 by the foregoing method of this invention. Injection well borehole 11 is preferably equipped with casing 23 cemented therein and sealed with cement. A tubing string 29 is disposed in well borehole 11 and packed off at packer 34. Conventional heating, pumping, heat exchanging and separating equipment are associated with well boreholes l1 and 12 for injecting fluid from well borehole 11 through perforations 31 in well borehole 11, through the permeable channel 25 created by intersecting fractures, such as 14 and 16, into well borehole 12 through perforations 33 therein. Well borehole 12 preferably is cased with casing 34 surrounded by cement 24. Since certain subterranean earth formations, such as oil and shale deposits in Colorado, Utah, and Wyoming, are practically impermeable except for certain natural vertical fractures, the method of this invention improves injectivity and fluid communication between two or more wells from a succession of vertical fractures.

We claim as our invention:

1. In a method of recovering liquefiable components from a normally impermeable subterranean earth formation comprising the steps of:

extending at least a pair of well boreholes into said earth formations;

forming generally vertical fractures extending along generally parallel paths from each of said pair of well boreholes;

injecting hot fluid through at least one of the well boreholes until flow into at least one of the fractures therein is thermally closed by the swelling-shut of the walls of said fracture;

pressurizing fluid in at least one well borehole in which at least one fracture has been thermally closed until at least one new fracture is formed; repeating the steps of injecting hot fluid and pressurizing fluid at successively higher temperatures and pressures until the resultant fractures form a channel interconnecting said well boreholes through which fluid flows from one well borehole to another; and

circulating hot fluid, by injection into one of said well boreholes opening into said fracture channel and production from another of said well boreholes opening into said fracture channel, at a temperature below that at which the last fracture was opened into the well borehole into which said hot fluid is injected but above that at which the last fracture was thermally closed within the well borehole into which said hot fluid is injected, so that most of the injected fluid is conveyed through said fracture channel from one well borehole to another.

2. The method of claim 1 including the step of adjusting the composition of said circulating hot fluid to the extent required to circulate fluid that removes solid materials from the walls of fractures interconnecting said well boreholes without a significant reduction in the'average rate of flow between the well boreholes so that the effective permeability of the fractures interconnecting said well boreholes is increased relative to that of other fractures.

3. The method of claim 2 including the step of increasing the temperature of the circulating fluid as the permeability of the interconnected fractures is increased, with the temperature being increased to the extent required to circulate fluid capable of liquefying the liquefiable components of the solid materials removed from the walls of the interconnecting fractures.

4. The method of claim 3 including the step of recovering hydrocarbons from the liquefiable components of the circulating fluid.

5. The method of claim 3 including the step of increasing the permeability of the interconnected fractures by injecting acid therethrough adapted to react with at least 1 portion of the rock matrix forming the walls of said fractures.

6. The method of claim 3 including the step of initiating an underground combustion Within said subterranean earth formation so as to effect recovery of petroleum materials.

7. The method of claim 2 wherein the step of adjusting the composition of said circulating fluid includes the step of incorporating acidic solid-material-removing components into said circulating fluid.

8. The method of claim 7 wherein the step of incorcirculating fields includes the step of incorporating nitric acid.

References Cited UNITED STATES PATENTS 2,813,583 11/1957 Marx et a1. 166-271 3,284,281 11/1966 Thomas 166-271 X 3,346,044 10/1967 Slusser 166259 X 3,379,250 4/1968 Matthews et a1 166-271 10 CHARLES E. OCONNELL, Primary Examiner I. A. CALVERT, Assistant Examiner US. Cl. X.R.

porating solid-material-removing components into said 15 166271, 272 

